save intermediate render result to OpenEXR not working (Cycles/ v.2.79)
If Blender is crashing during super high-resolution render for whatever reason, consider using add-ons for precise region render, like blender-renderborder or accurate-render-border (not sure about their 2.8 compatibility) or with some python usage. Enabling Crop in render settings can also help.

Then stich small chunks into one image in PS, Gimp or other image editor.